The present invention relates to a carbon material which intercalates into or deintercalates from lithium, and to a method for manufacturing the same. In particular, the present invention relates to a lithium secondary battery, which uses carbon material as a negative electrode active material, having a high energy density and a long life. The lithium battery is suitable for use in portable apparatus, electric automobiles, power storage, etc.
The Lithium secondary battery using lithium metal for the negative electrode has some problems relating to safety. For example, lithium easily deposits like dendrite on the lithium metal negative electrode during repeated charging and discharging of the battery, and if the dendritic lithium grows to a positive electrode, an internal short circuit will be caused between the positive electrode and the negative electrode.
Therefore, a carbon material has been proposed as the negative electrode active material in place of lithium metal. Charge and discharge reactions involve lithium ion intercalation into the carbon material and deintercalation from the carbon material, and so lithium is hardly deposited like dendrite. As for the carbon material, graphite is disclosed in JP-B-62-23433 (1987).
The graphite disclosed in JP-B-62-23433 (1987) forms an intercalation compound with lithium, because of intercalation or deintercalation of lithium. Thus, graphite is used as a material for the negative electrode of the lithium secondary battery. In order to use graphite as the negative active material, it is necessary to pulverize the graphite to increase the surface area of the active material, which constitutes a charge and discharge reaction field, so as to allow the charging and discharging reactions to proceed smoothly. Desirably, it is necessary to pulverize the graphite to powder having a particle diameter equal to or less than 100 μm. However, as is apparent from the fact that graphite is used as a lubricating material, the graphite easily transfers its layers. Therefore, its crystal structure is changed by the pulverizing process, and formation of the lithium intercalated compound might be influenced by undesirable effects of the pulverizing process. Accordingly, the graphite after the pulverizing process has a great deal of crystalline structural defects. In a case when graphite is used as an active material for the negative electrode of the lithium secondary battery, a disadvantage results in that a large capacity can not be obtained. Furthermore, preferable performances of rapid charging and discharging are not obtained because the lithium intercalation-deintercalation reaction is disturbed by the above defects.